The Expanding Options of Partial Hand Prostheses
Individuals with limb loss at the partial hand and partial finger levels, whether due to traumatic amputation or congenital deficiency, constitute the largest patient populations within the realm of upper-limb absence.
Despite their prevalence, prosthetic management alternatives at these distal amputation levels have historically been limited. However, in recent years, new prosthetic options have been developed to address this void. These have appeared within the models of traditional prosthetic care and may, in time, be supplemented by developments within less-traditional "disruptive technology" models of open-source software and 3D printing. While not exhaustive, this article provides an overview of the expanding range of partial finger and partial hand prosthetic options available. It's apparent that no single approach is universally better than the others, but that the correct clinical approach is patient specific and may often involve more than one device.
A long-standing option that remains a viable treatment consideration in cases where simple, robust construction and performance is desired is static opposition posts. While a modest anatomic appearance might be achieved with these applications, their primary focus is function over aesthetics. Given the broad range of amputation presentations that are encountered at these distal levels, there are a number of variations of the post concept. They are all based on the premise of leveraging a portion of the residual hand or finger against a fixed prosthetic element to mimic a grasping action or provide a stable opposing surface. Figure 1 illustrates the management of a thumb deficiency with the APRL two-position thumb, allowing both wide and narrow grip options. In a related approach, Partial Hand Solutions' M-Thumb is a more recently developed, commercially available static thumb option that can be passively positioned by the user in flexion, extension, and rotation to adapt to different gripping requirements. Figure 2 illustrates an alternate concept in which the prosthesis restores the palmar surface of the residual limb and transfers lifting loads from the sensitive residual fingers to the more tolerant dorsal surface of the forearm.
Another well-established treatment option that continues to find ideal application in the management of some partial hand and partial finger deficiencies is silicone restoration. In contrast to the robust constructions associated with opposition posts, the primary focus of silicone restoration is aesthetics rather than sturdy performance. Color matching the silicone to the skin of the residual limb is inherently challenging and requires careful communication with the silicone manufacturer. Patient expectations regarding color matching should be managed from the outset. While it can be close, it is rarely exact as physiologic skin tones will change throughout the day and the year according to a host of variables. Silicone restorations can range from fingertips, designed to replace deficits at or near the distal interphalangeal (DIP) joint (Figure 3), to larger glove-style restorations that extend proximal to the wrist joint and allow the remaining digits to escape as needed to preserve aesthetics and function (Figure 4). The compliant nature of silicone can offer the additional benefit of an added degree of cushioning to the sensitive distal aspects of the residual fingers and hand.
Bio-Mechanical Prosthetic Finger
For digit amputations at or near the DIP joint, Naked Prosthetics' (formerly RCM Enterprise) Bio-Mechanical Prosthetic Finger (BPF) represents a body-powered option that allows the user to regulate flexion and extension motion at the DIP joint through his or her movements at the proximal interphalangeal (PIP) joint. The linkage system, invented by Colin Macduff to address his own digit amputation, couples physiologic PIP flexion motion with prosthetic DIP flexion (Figure 5). Similarly, prosthetic DIP extension is created and controlled by physiologic PIP extension. Thus, in contrast to static opposition posts and silicone restorations, the user has the ability to actively control the movement and position of the prosthesis. In addition, the frame construction of the BPF provides a protective casing around the residual finger that shields the often sensitive digit from environmental impacts and pressures.
Developed by Matthew Mikosz, CP/L, an upper-limb prosthetics specialist, for digit amputations at or near the PIP joint, Partial Hand Solutions' Partial M-Finger provides body-powered prosthetic control of PIP joint movement. As with the BPF, prosthetic flexion is coupled to movement of the next most proximal joint segment as anatomic flexion at the metacarpophalangeal (MCP) joint creates prosthetic flexion at the PIP joint. This is done through a flexible linkage system in which a Spectra cable is run from the base of the Partial M-Finger, across the dorsum of the proximal phalanx, attaching proximal to the MCP joint (Figure 6). The tension of the Spectra cable determines the responsiveness of the system, or how readily prosthetic PIP joint motion occurs with MCP joint flexion. Because of the flexible nature of the cable-based linkage system, PIP joint extension is not directly coupled to MCP joint extension, but is driven by internal springs within the individual Partial M-Finger and occurs in the absence of dorsal cable tension.
The prosthetic finger pieces are available in three different lengths and are mounted to custom sockets, which are often fabricated from silicone for benefits the material offers, in particular the ability to cushion and protect the residual finger segments while ensuring an optimal fit.
The distal attachment of the cable is best obtained through a silicone sheath that rests on the dorsum of the hand but encircles the wrist distally. This provides a secure, comfortable attachment point without unduly restricting the motion of the remaining hand and wrist.
A closely related prosthetic option is Partial Hand Solutions' full M-Finger, designed for the management of digit absences at or near the MCP joint level. In the full M-Finger system, prosthetic articulation occurs at both the MCP and PIP joints. As with the Partial M-Finger system, flexion at these joints is created by tension from a Spectra cable with extension created by internal springs. In the M-Finger system, cable excursion is created by wrist flexion as the cable runs from the base of the M-Finger, across the dorsum of the hand, and is mounted proximal to the wrist joint (Figure 7). The responsiveness of the system is determined by cable tension and refined during the fitting process.
As with the Partial M-Finger, the immediate interface against the residual hand is generally provided in the form of a custom silicone socket to minimize any mobility restrictions and cushion the distal aspect of the residual hand. A laminated frame generally serves as the attachment point for the M-Finger itself, which is available in five different lengths.
Developed by Jorge Zuniga, PhD, and his research team at Creighton University, the Cyborg Beast is described as a "snap together" hand that requires sufficient "wrist movement and strength for proper function." It can be printed and assembled by interested individuals with access to a 3D printer. In contrast to the approaches described earlier with custom sockets and stock finger sizes, the Cyborg Beast is customized to individual patients through scaling the size specifications of the individual pieces prior to printing. This can be done according to a child's age or through an individual's anatomic measurement. Individual parts are printed and assembled according to openly accessible instructions.1 The body-powered control of this approach is similar to that described with the M-Finger system in that wrist flexion provides excursion through a cable system that mounts proximal to the wrist, runs along the dorsum of the hand, and inserts into the base of the individual prosthetic digits.
Described as a collaborative effort by some of the top designers within the e-NABLE 3D prosthetic hand community, the Raptor Hand (Figure 8) is designed with "ease of printing and assembly in mind." Unique features of this hand include 3D-printed assembly pins, obviating the need for Chicago screws, and a modular cable tensioning system. The system is further described as the product of "the best and most widely tested ideas from a year of crowd-sourced innovation." As with the Cyborg Beast, the Raptor is scaled to size based on anatomic measurement prior to printing and then assembled using printed parts and easily acquired accessory foams, cords, and hardware.2
The inclusion of the Cyborg Beast and Raptor Hand in this article should not be interpreted as an endorsement of these designs among certified prosthetists. While intriguing, these designs and others like them carry a number of concerns with regard to liability and U.S. Food and Drug Administration (FDA) compliance. As these devices affect the structure and function of the human body, they must ultimately be considered medical devices, subject to the oversight and ruling of the FDA.3 The unregulated, heterogeneous, and inconsistent nature of their designs, printing, assembly, and provision present obvious concerns with respect to quality control and consistency. Until their safety and efficacy are properly scrutinized and ensured, the provision of such systems creates liability risks to established prosthetics providers. However, the design concepts emerging within these 3D-printed devices and collaborations with the designers behind them will likely influence future partial hand prosthetic solutions.
i-limb digit and Vincent Finger
A transition from the body-powered systems described thus far to externally powered systems is represented with Touch Bionics' i-limb digit system (Figure 9) and Vincent Systems' Vincent Finger (Figure 10). While externally powered prosthetic hands have been commercially available for decades, externally powered digit systems have been challenged by their inherent spatial limitations. Current systems house the battery and additional components on the forearm to reduce the bulk of the operating system on the hand itself (Figure 10).
The advantages of externally powered systems are consistent with those seen in the management of more proximal upperlimb prosthetic systems. For example, patients are provided with substantially stronger grip strength with less physiological cost on the surrounding joints. Common control strategies include myoelectrodes (Figure 9, bottom), commonly positioned over the hypothenar and dorsal interossei muscle groups or force sensing resistors (FSRs) positioned adjacent to a mobile residual digit (Figure 10, top).
The length of the prosthetic digits at their base effectively limits candidacy for such systems to those with amputations at or near the MCP joints.
As partial hand and partial finger deficiencies constitute the most prevalent levels of upper-limb loss, it is encouraging to see the recent expansion of treatment options available to these populations. Once confined to functional opposition posts and aesthetic silicone restorations, prosthetic options now include a variety of body-powered approaches that span a range of amputation levels as well as externally powered, full digit systems. Concurrent with the emergence of crowd-sourced 3D-printed systems has been a growing general interest and awareness of prosthetic management of these once neglected amputation levels. As such, it is important for prosthetists to have a working familiarity with the growing body of available treatment options.
Phil Stevens, MEd, CPO, FAAOP, is in clinical practice with Hanger Clinic, Salt Lake City. He can be reached at .
- Fise, T. 2014. AOPA responds to liability and FDA compliance issues. O&P Almanac 63(11):24.